Methods for producing titanium-clad metal



July 25, 1961 F. c. KELLEY 2,993,269

METHODS FOR PRODUCING TITANIUM-GLAD METAL Filed Dec. 15, 1958 Titan/um iIron Tifanium 30% Reduction In venfor: Floyd 6. Kelley,

by )Q/ m His Afforney.

United States Patent "ice 2,993,269 METHODS FOR PRODUCING TITANIUM-GLADMETAL Floyd C. Kelley, Schenectady, N.Y., assignor to General ElectricCompany, a corporation of New York Filed Dec. 15, 1958, Ser. No. 780,6276 Claims. (Cl. 29-424) This invention relates to the provision of anadherent, corrosion-resistant coating of titanium on a different metalbase. More particularly, it is concerned with the provision of anadherent, corrosion resistant coating of titanium on a different metalbase by cold-rolling and the fabrication of formed structural elementsof titanium from such a composite material. This application is acontinuation-in-part application of application Serial No; 432,329, F.C. Kelley, now abandoned, assigned to the same assignee as the presentapplication.

The chemical and physical characteristics of substantially pure titaniumsuggest many advantages over other structural materials now in use.Unfortunately, some of the properties which render titanium desirablefor certain applications have in the past prevented its use. Forexample, no economical way has been previously known to consistentlyprovide a readily corrodible material, for example, iron, aluminumalloys, and the like with an adherent, corrosion-resistant, continuouscoating of titanium. Further, because of certain of the mechanicalproperties of titanium, it has been previously deemed impractical tofabricate complexly shaped structural elements of relatively thin-walledtitanium. My invention is concerned with methods of providing atitanium-clad base metal. i

A principal object of this invention is to provide a method of claddinga metal with titanium.

A further object of this invention is to provide coldrolled iron sheethaving an adherent, substantially continuous coating of titanium on itssurface.

Another object of this invention is to provide an aluminum body havingan adherent, substantially continuous coating of titanium on a surfacethereof.

Another object of this invention is to provide an aluminum alloy bodyhaving an adherent, substantially continuous coating of titanium on asurface thereof.

It is still a further object of this invention to provide a method ofmanufacturing formed structural elements of substantially pure titanium,and such elements made by said process.

Other objects and advantages of this invention will be in part obviousand in part explained by reference to the accompanying specification anddrawings in which:

FIG. 1 is a section through a titanium-clad iron body made according tothe present invention; and

FIG. 2 is a view showing a method for reducing the thickness of thetitanium-clad iron body to effect bonding between the two metals.

FIGS. 1 and 2 are also illustrative of other titaniumclad bodies inaccordance with the teachings of this invention, including for example,aluminum and its alloys.

Briefly stated, I have provided a method of cladding a base metal withan adherent, continuous thin layer of substantially pure titanium bycold rolling, a product produced by said process which has utility perse or as an intermediate which may be further fabricated and treated toproduce formed, thin-walled structural elements composed ofsubstantially pure titanium. invention and the various aspects thereof,minor variations in procedure and adaptations of the end products mayoccur to those skilled in the art; consequently, I do not intend theseveral examples set forth in the specification to be construed aslimitative but merely exemplary.

2,993,269 Patented July 25, 1961 Titanium metal is produced at presentby two processes, the so-called iodide process and the Kroll process. Inthe former, titanium iodide vapor is decomposed on a heated titaniumwire, depositing pure titanium on the wire. In the Kroll method,titanium chloride is treated with metallic magnesium to produce metallictitanium sponge and magnesium chloride. The sponge is then melted andcast into ingots. Because of the difiiculty of removing all themagnesium chloride from Kroll process titanium and due to the usualpresence of somewhat larger amounts of oxygen and nitrogen, it isslightly less pure than titanium produced by the iodide process. ASTMspecification places the following limits on high purity titanium withreference to the iodide process.

Percent Titanium minimum 99.9 Carbon maximum 0.03 Silicon do 0.02 Irondo 0.02 Aluminum do 0.03 Nitrogen do 0.01 Manganese do 0.04 Others dn0.01

A recent process called The Electrolytic Process also results in a pureform .of titanium which may be employed in this invention.

One of the most important properties of titanium from a commercial andengineering standpoint is its resistance to corrosion. While titanium ishighly reactive at elevated temperatures and will react readily withgases such as oxygen, nitrogen and hydrogen at temperatures above Inpracticing my 600 F., at room temperature, it is practically inert withrespect to these gases, oxidizing acids such at nitric and aqua regia,dilute sulfuric and hydrochloric acids and to most organic acids. It isresistant to dilute alkalies, as well. One of titaniums most spectacularproperties is its resistance to corrosion by solutions of chloride. Itis practically inert to sea water, to boiling dilute solutions of manychlorides and is unaffected by boiling saturated solutions of sodiumchloride.

As previously pointed out, titanium has a very high afiinity for certainof the gases present in the atmosphere at elevated temperatures. Forthis reason, it is very useful as a getter in vacuum apparatus. Itfunctions to remove residual molecules of atmospheric gases in evacuatedcontainers or systems in a manner well known to that art.

The singularly good resistance of titanium to corrosion by sea waterhas, of course, led to many attempts to clad steel and iron withtitanium for marine and other uses where such an environment isencountered. Hot rolling techniques have been attempted but because ofthe extreme activity of titanium at temperatures necessary for hotrolling, it has been necessary to resort to pack rolling. In this, atitanium slab is placed in juxtaposition to an iron slab to be coatedand the slabs enclosed in a gastight metal envelope which is then eitherevacuated or flushed with a noble gas. The pack is then heated,generally in a noble gas atmosphere furnace and hot rolled. In additionto the inherent difiiculties and expense of such a procedure, this typeof cladding has not produced consistently good bonding between the ironslab and the titanium.

Electrodeposition of titanium on iron has also been investigated. Thismethod has several limitations in that the condition of the surface tobe plated is critical, high bath temperatures are necessary and the highcurrent densities required make the process of doubtful practical value.

I have discovered that under particular conditions, an

adherent, continuous coating of titanium may be applied to iron by asimple cold-rolling procedure.

A first example of my invention deals with the production oftitanium-clad, cold-rolled iron upon which the titanium coating wasapplied to one side only.

A strip of cold-rolled iron 0.125 inch thick was annealed at about 1650F. in hydrogen. After annealing, one face of the sheet was grit blastedand wire brushed. A similar size strip of titanium 0.004 inch thick wasproduced by are melting iodide titanium in vacuum and cold rolling.After cold rolling, one of its faces was wire brushed. The two sheetswere placed with their cleaned faces together and the two sheets securedtogether. One method I have found suitable is to tack or spot weld thesheets together at one edge (the entering edge to the rolls) in a fewplaces, forming a composite sheet or work piece about 0.129 inch thick.This composite sheet was then cold reduced by a rolling mill having castiron working rolls. In order to prevent the titanium from welding to thechilled cast iron working rolls under rolling stresses, a protectivecoating was applied to the outer surface of the titanium layer.Alternatively, of course, such a protective coating could be applied tothe surface of the working roll. While many materials might be used forthis protective coating, I have found commercial milk of magnesia, asuspension of magnesium hydroxide, Mg(OH) in water, satisfactory forthis purpose. A preferred method is to paint a thin layer of milk ofmagnesia on the titanium surface to be worked and permit the layer todry, forming a substantially continuous coating. The compositetitanium-iron strip or sheet was then cold reduced in one pass fromabout 0.129 inch to about 0.070 inch, a reduction of about 46 percent.The titanium was found to be securely welded to the iron after thisinitial pass. The composite sheet was then given a second heavyreduction in a single pass, reducing it to about 0.043 inch, or about 39percent reduction. The composite sheet was then cold rolled in severalpasses to 0.005 inch sheet, amounting to a total cold reduction of 96percent.

Upon careful examination, the titanium coating was found to be adherent,continuous and about 0.00015 inch thick. There was no indication ofcracking, either along the edges or at any other portion of the surface,except for minor defects in the zones of the spot welds. These, however,were limited to the extreme edge and were removable by the normaltrimming or slitting operation following rolling.

I have discovered that vacuum-annealed iodide titanium cold-rolled to0.004 inch thick strip may be used in the foregoing example in place ofthe arc-melted titanium without adversely affecting the quality of thefinished material.

I further subsequently discovered that when steel working rolls are usedin place of chilled cast iron, the titanium does not tend to weld to theroll surface, rendering coating unnecessary.

Using a similar procedure, I have discovered that iron strip may besuccessfully clad on both sides simultaneously, by the expedient ofstarting with a composite strip of titanium-iron-titanium. In this case,of course, the protective coating must be applied to the titaniumcomprising both faces if cast-iron rolls are used.

In both of the above-cited examples, annealing was found unnecessary atany stage of the rolling. If for any reason annealing should be deemednecessary, it should be done in either a vacuum or an atmosphere inertto titanium at elevated temperatures.

As further demonstration of the tenacity and continuity of the titaniumcoating produced in this manner, cups about 0.125 inch in diameter andabout 0.150 inch deep were cold drawn from the 0.005 inch titanium:

ing a complete, .integral, self-sustaining cup of coldworked titaniumhaving no observable holes or other structural defects. The iron may beremoved in any suitable manner, for example, by means of dilutehydrochloric acid. The wall thickness of the titanium elements thusobtained was of the order of,0.00015 inch or less. Obviously thethickness may be varied depending upon the initial dimensions of thework piece and by the amount of working. The form of the resultingtitanium element is not restricted to cup-form, nor to deep drawingoperations. It is, therefore, apparent that structural elements ofsubstantially pure titanium may be conveniently made by forming thedesired shape from the clad composite material by drawing, bending,machining or any suitable process and then dissolving the base materialfrom the titanium. In this manner, complexly shaped strong structuralelements of very thin titanium metal may be economically mass produced.Elements of this type are suitable for use as load bearing elements inevacuated apparatus. Complexly shaped electrodes, heating elements orthe like, may advantageously be made from thin-walled titanium in thismanner. By providing means for heating these elements, eflicientgettering may be provided over long periods of time, and any tendencyfor the evacuated apparatus to become gassy may be effectivelycountered.

I have further discovered that the successful practice of this processrequires substantially pure titanium. When iodide titanium was usedconsistently good results were obtained. On the other hand, when Krollprocess titanium was used, the titanium did not consistently weld to theiron satisfactorily during the first heavy pass, and during subsequentreduction, the titanium adhered to the iron only in a few places andseparated in others to form a lace-like, non-continuous texture whichexposed the iron base through its openings. Kroll process titanium isknown to be somewhat less ductile than iodide titanium. Presumably inthis case, the iron deformed more readily than the titanium and pulledthe relatively thin titanium layer apart in zones of non-adherence.

Additionally, I have discovered that because titanium tends to workharden and thus lose ductility, it is important that the first passthrough the working rolls accomplishes a large reduction. Indifferent orpoor adherence and continuity result unless a heavy first draft of atleast 30% or more is taken. Therefore, I regard such a drastic initialreduction to be an important feature of my invention.

Another example of the teachings of this invention, as relating totitanium-clad iron bodies, is titanium cladding of aluminum and aluminumalloy bodies. The requirements leading to good results in claddingaluminum generally, are similar to those previously described incladding iron. It is essential that the aluminum or aluminum alloy haveabout the same ductility and hardness as the iodide titanium to preventrupturing of the titanium during the rolling process. Good results wereobtained using Duralumin and magnesium aluminum alloys with similarductility and hardness to titanium. Such a com crackingobserved. .Theiron was then dissolved, leavposite structure exhibits excellentcharacteristics for a wide variety of uses. By way of example, sincetitanium is generally not attacked by various foods, and since aluminumis an excellent heat conductor, pot, pans and food-containing utensilsgenerally, represent but one of many applications. One preferredcladding method is descrbied in the following example. A sheet ofDuralumin, /2 x 2 inch, in the as received condition was heated to about500 C. in a hydrogen atmosphere and water quenched. The Duralumin sheetwas then wire brushed on one side to remove the aluminum oxide coating.A titanium sheet also /2 x 2 inch was also wire brushed on one side. Thetitanium sheet was 0.050 inch thickand the Duralumin sheet was 0.062inch thick. The wire brushed surfaces of the titanium and the aluminumsheets were then placed in contiguous relationship,

the outer surfaces coated with Mg(OH) in water as previously describedin the iron cladding method, and passed through a pair of 10 inchdiameter reduction rolls for an overall thickness reduction of about40%. Rolling velocity was 100 feet per minute. After the first draft ofabout 0.050 inch, the two strips were found to be firmly bonded togetherand capable of further mechanical Working. The final thickness of thetitanium aluminum composite structure was about 0.055 inch. Of thisthickness, 0.030 inch represented titanium. A second draft of the joinedsheets of about 0.020" resulted in some overworking of Duralumin at theedges of the strip but the remainder was firmly bonded. While a 30%initial reduction is suflicient as a minimum, initial reduction of 40 to50% is more satisfactory for aluminum and titanium.

Duralumin is a trade name applied to the first aluminum-copper-magnesiumtype of age hardenable alloy (178), which contains nominally 4% Cu., /2%Mn and /2 Mg. It is commercially available as Duralumin.

The above example was also employed in cladding a magnesium aluminumalloy, MgAl, commercially available and referred to as 6061T containing:

(14-08% Si, 0.7% Fe, 0.150.40% Cu, 0.15% Mn, 0.8-l.2% Mg, 0.l0.35% Cr,0.25% Zn, 0.15% Ti, ODS-0.15% impurities,

and the remainder aluminum.

A further method of cladding using press apparatus employed the sameprocess as given in the above example with a sample of titanium of 0.004inch thick and 2S (commercially pure) aluminum 0.010 inch thick. Insteadof passing these sheets through rolls, they Were placed in a hydraulicpress and pressure of about 100 tons per square inch exerted thereon.After application of this pressure, the titanium and aluminum werebonded as a composite structure.

It is preferable to anneal the composites before further mechanicalWorking. Stress relieving by heating to 300 C. gives good results.

I have, therefore, discovered that metallic titanium may be clad on adifferent metal base by cold-rolling provided the titanium will weldunder plastic deformation to the base metal and further, that theductility of the titanium is suflicient to permit its plasticdeformation at a rate compatible to the deformation of the base metal.Further, the titanium should work harden at approximately the same rateas the base metal so that the two metals continue to elongate at aboutthe same rate during all stages of working. The foregoing explanationadvanced for the behavior of titanium with respect to cold roll claddingis intended to express a probable theory and should be regarded as such.

From the foregoing, it is apparent that I have discovered a practicalmethod of applying an adherent, continuous coating of titanium on adifferent metal base by cold rolling, and more particularly, I havesucceeded in producing cold-rolled iron strip material having a coatingof metallic titanium. I have also discovered a novel, practical means offabricating formed structural elements composed of thin titanium.

The specific examples have been set forth as illustrative of theinvention, it being understood that many changes and modifications maybe made without departing from my invention in its broader aspect, and Iaim, therefore, in the appended claims to cover all such changes andmodifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. A method of forming a thin-walled titanium article comprising incombination, utilizing a thin sheet of high-purity titanium of at leastabout 99.9% titanium and a sheet of metal taken from the groupconsisting of iron and aluminum, cleaning at least one surface of eachof said sheets to remove foreign matter, placing the sheets in assembledrelationship with the cleaned surfaces in contact with each other,effecting at least a 30% cold reduction in the thickness of the joinedsheets by running them through opposed rolls to bond said sheetscontinuously over their cleaned and engaging surfaces to thus unite thesheets into an integral body capable of withstanding further mechanicalworking and remaining integral, forming the joined sheets by furthermechanical working into a configuration other than sheet form, andthereafter removing the other metal from the titanium.

2. A method of forming a bond between a sheet of titanium and a sheet ofmetal taken from the group consisting of iron and aluminum comprising,selecting a relatively thin titanium sheet of a purity of about 99.9%titanium and a sheet of metal from said group having similar workhardening and ductility characteristics to that of titanium, cleaning aportion of each of said sheets to remove foreign matter, placing thecleaned portions of said sheets in assembled and contiguousrelationship, effecting at least a 30% cold reduction in the combinedthickness of the joined sheets in one pass through a rolling mill toprovide a bond extending substantially continuous between the cleanedand engaging portions so that said sheets become integral and capable ofwithstanding a further mechanical working while remaining integral.

3. The method as described in claim 2. as applied to bonding of titaniumand aluminum.

4. The method as described in claim 2 employed to bond titanium and analuminum alloy.

5. The method as described in claim 2 wherein said 40% reduction isaccomplished in a press apparatus instead of a rolling mill.

6. A method of forming a bond between a relatively thin sheet oftitanium and a relatively thin sheet of iron comprising utilizing saidsheet of titanium of at least 99.9% titanium, cleaning at least onesurface of each of said sheets to remove foreign matter, placing thesaid sheets in assembled relationship with the cleaned surfaces incontact, and effecting at least 30% cold reduct1on in the thickness ofthe joined sheets by running them through opposed rolls to provide abond between said sheets extending continuously over the cleanedportions in engagement so that said sheets are united into an integralbond capable of withstanding further mechanical working and remainingintegral.

UNITED STATES PATENTS References Cited in the file of this patent2,691,815 Boessenkool Oct. 19, 1954

1. A METHOD OF FORMING A THIN-WALLED TITANIUM ARTICLE COMPRISING INCOMBINATION, UTILIZING A THIN SHEET OF HIGH-PURITY TITANIUM OF AT LEASTABOUT 99.9% TITANIUM AND A SHEET OF METAL TAKEN FROM THE GROUPCONSISTING OF IRON AND ALUMINUM, CLEANING AT LEAST ONE SURFACE OF EACHOF SAID SHEETS TO REMOVE FOREIGN MATTER, PLACING THE SHEETS IN ASSEMBLEDRELATIONSHIP WITH THE CLEANED SURFACES IN CONTACT WITH EACH OTHER,EFFECTING AT LEAST A 30% COLD REDUCTION IN THE THICKNESS OF THE JOINEDSHEETS BY RUNNING THEM THROUGH OPPOSED ROLLS TO BOND SAID SHEETSCONTINUOUSLY OVER THEIR CLEANED AND ENGAGING SURFACES TO THUS UNITE THESHEETS INTO AN INTEGRAL BODY CAPABLE OF WITHSTANDING FURTHER MECHANICALWORKING AND REMAINING INTEGRAL, FORMING THE JOINED SHEETS BY FURTHERMECHANICAL WORKING INTO A CONFIGURATION OTHER THAN SHEET FORM, ANDTHEREAFTER REMOVING THE OTHER METAL FROM THE TITANIUM.